Definition
A method of radio communication in which transmission and reception occur on the same frequency, allowing only one station to transmit at a time while the other listens.
Plain English
Both people use the same frequency to talk to each other, but only one person can talk at a time. While one is talking, the other can only listen.
Context Anchor
Seen in aircraft communication radio discussions, especially when describing normal VHF radio use between an aircraft and ATC or another station.
Derivation
From Latin 'simplex' meaning 'single' or 'one-fold.' Here it points to the single shared frequency used for both sending and receiving.
Why Pilots Care
Most aviation radio work is simplex. Knowing this is why you must wait for the other station to finish before keying the mic — stepping on a transmission means neither party hears it clearly.
Analogy
Like a walkie-talkie: you press the button to talk, release it to listen, and only one person can speak at a time.
Intuition Check
Simplex does not mean the radio system is easy or basic. Here it means the same frequency is used for both talking and listening.
Example Sentence 1
Pilot-to-tower communication is a simplex operation, so the pilot waits for the controller to finish before responding.
Example Sentence 2
When switching to a simplex frequency for a remote airport, remember that only one aircraft can transmit at a time.